1. Field of the Invention
The present invention relates to a radio-controlled timepiece that corrects time based on time information that the timepiece received in radio waves.
2. Description of the Prior Art
A radio-controlled timepiece that receives radio signals indicating a precise time and automatically corrects time has been developed previously.
Currently, in various countries (e.g. Germany, Japan, America, and England) a radio station placed at a given location sends out a standard low frequency radio wave with time information superposed thereon. In Japan, two transmitter stations send out 40-kHz and 60-kHz standard low frequency radio waves modulated in amplitude by a time code of a format as in FIG. 5.
A radio-controlled timepiece, which receives the standard low frequency radio waves and corrects time data of a clock circuit, has been commercialized. In the case of a radio-controlled timepiece like this, it is equipped with an antenna and a tuning circuit for receiving a standard radio wave.
FIG. 4 is a block diagram of a conventional radio-controlled timepiece including a timing circuit. In FIG. 4, 401 represents an antenna which catches a radio signal to generate a received signal. 402 represents a tuning circuit in which capacitors are connected in parallel. And, the antenna circuit is tuned with the inductance value of the antenna 401 and the capacitance value of the tuning circuit 402 to e.g. a receiving frequency of 40 kHz. Signals from the antenna circuit tuned to 40 kHz are input to the receiving unit 403, and a time code including current time information is output.
The time format-recognizing unit incorporated in the control unit 404 picks out current time information from the time code, and the current time of the clock unit provided in the control unit 404 for clocking is corrected according to the precise current time information. The display unit 405 displays the time that the clock unit keeps (precise time after the correction in this case).
The control unit 404 performs ON/OFF control of the operation to receive a standard radio signal by performing ON/OFF control of the receiving unit 403.
Further, in recent years, as for example in a radio-controlled timepiece capable of receiving standard low frequency radio waves in Japan and Germany, a method including connecting two or more capacitors and switching them for tuning to the respective frequencies, JJY of Japan, 40 kHz and 60 kHz, and DCF of Germany, 77.5 kHz, has been adopted (see JP-A-2002-82187).
For adjustment of the receiving sensitivity of the tuning circuit in the above configuration, a method including preparing a few kinds of capacitors as the ones used in the tuning circuit and determining the capacitance of the capacitor so as to match each inductance of the bar antenna has been adopted.
In the method to adjust the receiving sensitivity, the way to exchange the capacitance of the capacitor has the following problem. That is, the adjustment performed by a combination of two or more capacitors makes the work to assemble the radio-controlled timepiece complicated and requires a long time and as such, the rate of work is unfavorable.
In addition, the bar antenna is relatively compact and has a high Q value and as such, only a slight change of factor, such as change of temperature or the presence of a metal near the antenna, can change the tuning frequency largely. Therefore, the method to adjust the receiving sensitivity has the following problem. That is, even when the adjustment is made to bring the tuning circuit into a tuning condition once, there is a high probability that tuning circuit falls into an untuned condition after having passed into the hands of a user, which deteriorates the receiving sensitivity.
The invention is made to solve the foregoing problems, and its subject is to facilitate tuning to a standard radio signal.
Also, a subject of the invention is to enable tuning to two or more standard radio signals different in frequency.
In addition, a subject of the invention is to allow the assembling work to be performed readily.